An optical buffer stores and holds an optical data stream for a fixed duration without conversion to electrical format, an important role in all-optical information processing. In order to perform buffering on chip in a small footprint, a great reduction in group velocity is necessary. Slow light schemes have been proposed using both optical and electronic resonances. Optically, the forward propagation of light is delayed by circulating it through many round trips in a resonator, which may be in the form of a ring resonator or as a defect mode in photonic crystal. This delay can be extended by simply cascading many resonators together, as in coupled resonator optical waveguides (CROW) and side-coupled ring resonator structures that generally employ more than 10 cavities. Electronically, the group velocity is greatly reduced by means of electromagnetically induced transparency (EIT), which is the result of destructive quantum interference between two coherently coupled atomic energy levels. A very large delay, but associated with a very narrow bandwidth, has been demonstrated experimentally at very low temperatures. Such a remarkable phenomenon causes a growing interest in optically mimicking EIT using two coupled resonators, whose coupled resonances resemble the two energy levels in an atom, but without the limitation of low temperature which has been a major hurdle for electronic resonance. This method is interesting because it is a relatively simple configuration that can produce a large delay.
Ideally, an optical buffer should have not only a large delay, but the delay should be constant over a broad bandwidth with low insertion loss. However, causality dictates that there is a constant delay-bandwidth product determined by the physical mechanism underlying the delay. The delay-bandwidth product is a measure of the number of bits that can be stored in the buffer (NST). For buffers based on resonators the delay-bandwidth product is typically less than one. For example, in the simplest configuration of an all-pass filter (APF) which consists simply of one ring coupled to one bus, the delay-bandwidth product is given byNST(APF)=τΔf=(1+r)/π√{square root over (r)}≅2/π<1,  (1)where Δf is the normalized full-width half-maximum (FWHM), τ is the maximum delay at the resonance, and r is the coupling coefficient between the waveguide and the ring. Note that here FWHM is used only for convenience, and the usable bandwidth is actually less than that due to the presence of higher order dispersion, thereby reducing the NST. In the case of 56 APF, for example, the system should be able to buffer 56×2/π=35 bits based on FWHM, but in reality it only buffers undistorted 10 bits which means the usable bandwidth is actually smaller.
Simply cascading the resonators does not necessarily increase NST. For example, the delay-bandwidth product of a CROW structure theoretically is given by NST≅N/2π, where N is the number of resonators. However, experimentally a CROW with N=100 has only achieved undistorted buffering for one bit. This is because while the delay is increased N-fold by using N resonators, the delay-bandwidth product remains about the same as that for a single ring since the pass-band also has a ripple profile with N peaks which inevitably imposes severe distortions in the signal and effectively diminishes the operating bandwidth by 1/N. The ripple, however, can be removed when the waveguide loss is high or when the coupling between the bus waveguide and the ring is different from that between the rings. This has been demonstrated where a delay of ˜110 ps combined with ˜17 GHz bandwidth is achieved with 12 coupled rings and a waveguide loss of ˜17 dB/cm. This corresponds to NST of 1.87, which is close to the value of (12/2π)=1.9 given by the theoretical estimate. This larger NST, however, is compromised by a high insertion loss of about 30 dB.